Prior art optical fiber communication cable includes four distinct generic constructions or arrangements of the electrical, optical, and mechanical elements to achieve the required respective performance functions. In the first approach the optical fibers are placed into a system of polymeric tubes cabled about a central strength member core, or into a radial system of chambers in the form of a helix about a central steel strength member. In a second generic approach, the fibers are encapsulated into a polymeric matrix which is enclosed within a composite system comprised of steel strength member wires and a relatively large diametered, thick-walled tubular copper conductor. The steel wires and the tubular copper conductor are always adjacent and in contact. The third generic approach attempts to incorporate optical fibers into existing electromechanical cable structures by providing a tight-buffer; i.e., added strengthening and jacketing of the individual fibers, such that they may be handled and processed on an equal footing with insulated electrical conductors. This method is known as the hybrid design approach. The fourth and most recent generic approach utilizes a copper alloy tube which is improved with respect to providing a smaller diameter for encapsulation of the optical fibers and placement of the synthetic fiber strength member in an outermost annular region of the cable cross-section.
The first generic type of cable configuration is exemplified by the U.S. Pat. No. 4,143,942 (Anderson, dated 3/13/79) and U.S. Pat. No. 4,199,224 (Oestreich, dated 4/22/79). In the former patent, a fiber or a multiplicity of fibers are enclosed within a polyacrylonitrile sheath, and as many as six of these units are in turn cabled about a central, synthetic fiber (i.e., KEVLAR/du Pont) strength member core. No electrically conductive path is provided. In the latter patent (#4,199,224) the cabled bundle is replaced by a radial system of chambers that are helical extruded and appear as radial "ribs" in the transverse cable cross-section. One or more optical fibers are placed in the chambers formed by the helical ribs which are formed directly over the central steel wire strength member. Insulated electrical conductors are placed in a layer over the system of chambered fibers, then covered by polymeric tapes and an external jacket.
The second generic type of optical fiber communication cable is represented by the Mondello U.S. Pat. No. 4,156,104 (5/22/79) and the Parfree and Worthington U.S. Pat. No. 4,239,336 (12/16/80), intended for use as optical fiber submarine cables. In the former patent (#4,156,104) the fibers are captured in an annular region of extruded elastomer around a central steel "kingwire." A maximum of twelve fibers are thus enclosed in an approx 0.102" overall diameter (OD). The latter is surrounded by at least two layers of unidirectional steel stranding, which is jacketed with a copper tube pulled down over the steel strand and swaged into the outermost interstices of the outer layer of steel wires to obtain positive mechanical contact. The latter is covered with an electrical grade dielectric of polyethylene to 0.827" OD. This extrusion represents the only physical/mechanical protection for the electro-optic functions of the cable. Similarly, the Parfree and Worthington fiber optic cable invention (#4,239,336) contains a composite steel strength member and thick-walled tubular electrical conductor adjacent to each other and enclosing a number of optical fibers along with a polymeric filler material in the center of the cable. In contradistinction to the Mondello cable invention, a dual-system of relatively large diametered, thick-walled copper tubes is formed in two processing operations over the optical fibers, and the resulting unit is surrounded with two contrahelical layers of steel wire strength members in direct contact with the outer surface of the tubular electrical conductor. This composite tubular conductor is then extruded with an electrical grade of polyethylene to a diameter greater than 0.850 inches. Again, the latter extrusion represents the only protection for the electro-optic functions of the cable.
Several problems have been recognized from actual experience with optical fiber submarine cable constructed as just described. One major problem concerns the vulnerability of the cable to sharkbite, abrasion, and anchor dragging. All of the cable manufactured previously has now been provided with additional copper alloy tube shielding over the dielectric and an outer jacket of high-density polyethylene to a diameter over 1.00". The cable diameter reduced correspondingly the total continuous length of cable that could be carried by the cable laying ship. There is now great economic and strategic interest in defining a fiber optic submarine cable that provides both improved survivability and a reduced transportation volume; for example, it is desired to provide a cable with diameter .ltoreq. 0.500 inches.
Prior art in fiber optic tow cables and ROV umbilical cables utilized variations of traditional EM cables to incorporate tight-buffered optical fiber elements on an equal footing with insulated electrical elements into the cable structure. A tight-buffer implies the fiber optics are individually strengthened and jacketed with various synthetic yarns, or steel wires, or composite glass/epoxy directly over and in contact with the primary/secondary buffer on the "as manufactured" optical fiber. (For a review of the pertinent methods and associated diameters for the individual fiber units see "Small-Diameter, Undersea, Fiber Optic Cable", T. Dohoda and T. Stamnitz, Proc. DOD Fiber Optics '88, McClean, Va., 23 Mar. 88; for a review of the generic ROV and tow cable configurations incorporating these units see "Fiber-Optic Tether Cable for ROV's", Proc. DOD Fiber Optics '88, McClean, Va. 23 Mar. 88; for a historical view on placing fiber optics into EM tow cable and ROV applications see, "Development and Design of Underwater Cable", T. Stamnitz, Sea Technology, Vol. 25, No. 7, pg. 29-33, July 1984).
While the hybrid approach for incorporating optical fibers into EM cables proved fruitful for the transmission of digital optical data, a growing need became apparent for a true electroopto-mechanical tow cable or ROV umbilical having a reduced diameter to reduce hydrodynamic drag forces. At the same time, a requirement for a large number of fibers within a small diameter tow cable has developed. This same requirement cannot be satisfied by fibers having the tight-buffered configuration.
The fourth and most recent generic configuration for an undersea, fiber optic cable is disclosed in U.S. Pat. No. 4,763,981issued to G. A. Wilkins (8/16/88). Based upon a relatively smalldiametered copper tube for encapsulation of the optical fibers, Wilkins achieves the potential for a significant cable diameter reduction, while providing a synthetic strength member outside the dielectric to serve as armoring protection for the electrooptic functions of the cable. The copper alloy tube element, unfortunately, could not be manufactured reliably and profitably such that this product is not available at the present time. In addition, requirements have arisen for undersea tow cables and long-haul submarine cables that require more fibers than can be provided in the Wilkins approach, and require a greater specific gravity than can be obtained using synthetic strength members.
Currently a need is recognized for the development of fiber optic cables having the capability to preserve optical phase and/or polarization data during the transmission between signal source points and remote monitoring (sink) points. This need becomes more acute when fiber optic sensors systems and coherent fiber communications systems must operate in an undersea environment, since transmissions must be made from various depths, and particularly, from extreme depths to signal processors at sea level. A difficult problem associated with fiber optic sensor technology concerns the high degree of sensitivity of the "downlead" optical fiber (contained in the connecting cable), which must apparently be exposed to the environmental parameter to be measured. For example, in the case of fiber optic interferometric hydrophones, optical fiber leads are used to interconnect the undersea sensor array to a remote processing sink in order eliminate need for optical-electronic conversion and the electrical transmission of collected data through long electro-mechanical (EM) cables. The latter EM cables have the disadvantage of insufficient bandwidth, excessive losses and/or excessive diameter and weight. On the other hand, use of polarization preserving fiber or special low birefringent fiber is usually unaffordable. Furthermore, it is difficult in hybrid fiber optic cable design which incorporates standard telecom fiber to protect the fiber from exposure to the undersea acoustic environment. Further, it is more difficult to prevent an increase in background phase noise induced by dynamic mechanical stresses associated with the operational tow environment. Thus, a continuing need exists in the state of the art for a reduced diameter electro-opto-mechanical cable configuration that incorporates a large number of fibers capable of preserving optical phase and/or polarization data during transmission through the fibers, while simultaneously transmitting electrical power and withstanding the physical demands of an undersea tow cable environment or a dynamic seafloor environment.